Testosterone, the major circulating androgen in men, is synthesized from cholesterol. The approximately 500 million Leydig cells in the testes secrete more than 95% of the 6-7 mg of testosterone produced per day. Two hormones produced by the pituitary gland, luteinizing hormone (“LH”) and follicle stimulating hormone (“FSH”), are required for the development and maintenance of testicular function and negatively regulate testosterone production. Circulating testosterone is metabolized to various 17-keto steroids through two different pathways. Testosterone can be metabolized to dihydrotestosterone (“DHT”) by the enzyme 5α-reductase or to estradiol (“E2”) by an aromatase enzyme complex.
Testosterone circulates in the blood 98% bound to protein. In men, approximately 40% of the binding is to the high-affinity sex hormone binding globulin (“SHBG”). The remaining 60% is bound weakly to albumin. Thus, a number of measurements for testosterone are available from clinical laboratories. The term “free” testosterone as used herein refers to the fraction of testosterone in the blood that is not bound to protein. The term “total testosterone” or “testosterone” as used herein means the free testosterone plus protein-bound testosterone. The term “bioavailable testosterone” as used herein refers to the non-SHBG bound testosterone and includes testosterone weakly bound to albumin.
The following table from the UCLA-Harbor Medical Center summarizes the hormone concentrations in normal adult men range:
TABLE 1Hormone Levels in Normal MenHormoneNormal RangeTestosterone298 to 1043 ng/dLFree testosterone3.5 to 17.9 ng/dLDHT31 to 193 ng/dLDHT/T Ratio0.052 to 0.33DHT + T372 to 1349 ng/dLSHBG10.8 to 46.6 nmol/LFSH1.0 to 6.9 mlU/mLLH1.0 to 8.1 mlU/mLE217.1 to 46.1 pg/mL
There is considerable variation in the half-life of testosterone reported in the literature, ranging from 10 to 100 minutes. Researchers do agree, however, that circulating testosterone has a diurnal variation in normal young men. Maximum levels occur at approximately 6:00 to 8:00 a.m. with levels declining throughout the day. Characteristic profiles have a maximum testosterone level of 720 ng/dL and a minimum level of 430 ng/dL. The physiological significance of this diurnal cycle, if any, however, is not clear.
Male hypogonadism results from a variety of patho-physiological conditions in which testosterone concentration is diminished below the normal range. The hypogonadic condition is sometimes linked with a number of physiological changes, such as diminished interest in sex, impotence, reduced lean body mass, decreased bone density, lowered mood, and decreased energy levels.
Researchers generally classify hypogonadism into one of three types. Primary hypogonadism includes the testicular failure due to congenital or acquired anorchia, XYY Syndrome, XX males, Noonan's Syndrome, gonadal dysgenesis, Leydig cell tumors, maldescended testes, varicocele, Sertoli-Cell-Only Syndrome, cryptorchidism, bilateral torsion, vanishing testis syndrome, orchiectomy, Klinefelter's Syndrome, chemotherapy, toxic damage from alcohol or heavy metals, and general disease (renal failure, liver cirrhosis, diabetes, myotonia dystrophica). Patients with primary hypogonadism show an intact feedback mechanism in that the low serum testosterone concentrations are associated with high FSH and LH concentrations. However, because of testicular or other failures, the high LH concentrations are not effective at stimulating testosterone production.
Secondary hypogonadism involves an idiopathic gonadotropin or LH-releasing hormone deficiency. This type of hypogonadism includes Kallman's Syndrome, Prader-Labhart-Willi's Syndrome, Laurence-Moon-Biedl's Syndrome, pituitary insufficiency/adenomas, Pasqualini's Syndrome, hemochromatosis, hyperprolactinemia, or pituitary-hypothalamic injury from tumors, trauma, radiation, or obesity. Because patients with secondary hypogonadism do not demonstrate an intact feedback pathway, the lower testosterone concentrations are not associated with increased LH or FSH levels. Thus, these men have low testosterone serum levels but have gonadotropins in the normal to low range.
Hypogonadism may be age-related. Men experience a slow but continuous decline in average serum testosterone after approximately age 20 to 30 years. Researchers estimate that the decline is about 1-2% per year. Cross-sectional studies in men have found that the mean testosterone value at age 80 years is approximately 75% of that at age 30 years. Because the serum concentration of SHBG increases as men age, the fall in bioavailable and free testosterone is even greater than the fall in total testosterone. Researchers have estimated that approximately 50% of healthy men between the ages of 50 and 70 have levels of bioavailable testosterone that are below the lower normal limit Moreover, as men age, the circadian rhythm of testosterone concentration is often muted, dampened, or completely lost. The major problem with aging appears to be within the hypothalamic-pituitary unit. For example, researchers have found that with aging, LH levels do not increase despite the low testosterone levels. Regardless of the cause, these untreated testosterone deficiencies in older men may lead to a variety of physiological changes, including sexual dysfunction, decreased libido, loss of muscle mass, decreased bone density, depressed mood, and decreased cognitive function. The net result is geriatric hypogonadism, or what is commonly referred to as “male menopause.” Today, hypogonadism is the most common hormone deficiency in men, affecting 5 in every 1,000 men. At present, it is estimated that only five percent of the estimated four to five million American men of all ages with hypogonadism currently receive testosterone replacement therapy.